The project focuses on the genetic and cytological organization of a small segment of the X chromosome of Drosophila. The objectives are to obtain mutant representatives of every gene in the region extending from 3A to 3C of the polytene map. Mutants are then characterized by complementation, recombination mapping and positioning relative to chromosomal aberration break points. Gene numbers and sizes are determined. Those chromosomal segments in which breakage may occur without causing detectable modifications of gene function are also mapped.